Hi, not so long ago, I had one of these rattle cans with zinc based paint for galvanise repairs / touch-up, and like has been said, very often was used just for one or two small areas to paint and although turning turning upside down each time, the nozzle blocked. After cleaning out the nozzle and trying again, it just kind of spat a few drops of paint and then stopped again, so I tried a brand new nozzle from an unused can and it still just spat a few drops and stopped. I think the can only had about a quarter of it's contents left but this paint was a bit pricy, so I was loathed to discard it, to cut a long story short, it turned out that it would only spray the paint when shook and then turned upside down and sprayed, but I only got short burst of spray each time and it never did spray the correct way up again, but at least the paint wasn't wasted, but seemed very strange why it wouldn't spray in the correct manner.
Regards Nick.